Filter out extended key codes

SDL2 has no mapping for extended keycodes, so SDL2 apps/games are not expecting them.

If an extended key code is encountered, the key code will instead be derived from the scancode, and the standard scancode will be returned for commonly remapped keys if no mapping is found, which matches the existing mapping and behavior of the real SDL2.

src/sdl2_compat.c

@@ -1281,6 +1281,24 @@ static SDL2_Scancode SDL3ScancodeToSDL2Scancode(SDL_Scancode scancode)
     }
 }
 
+static SDL_Keycode SDL3KeycodeToSDL2Keycode(SDL_Scancode scancode, SDL_Keycode keycode)
+{
+    /* Keys without the extended mask are passed through. */
+    if (!(keycode & SDLK_EXTENDED_MASK)) {
+        return keycode;
+    }
+
+    /* SDLK_TAB is an ASCII value and can't be converted directly from a scancode. */
+    if (keycode == SDLK_LEFT_TAB) {
+        return SDLK_TAB;
+    }
+
+    /* Convert the scancode directly to the keycode. This matches the mapping behavior
+     * of SDL2 backends unless some very esoteric key remapping is being used.
+     */
+    return SDL_SCANCODE_TO_KEYCODE(scancode);
+}
+
 /* (current) strategy for SDL_Events:
    in sdl12-compat, we built our own event queue, so when the SDL2 queue is pumped, we
    took the events we cared about and added them to the sdl12-compat queue, and otherwise
@@ -1322,7 +1340,7 @@ Event3to2(const SDL_Event *event3, SDL2_Event *event2)
     case SDL_EVENT_KEY_DOWN:
     case SDL_EVENT_KEY_UP:
         event2->key.keysym.scancode = SDL3ScancodeToSDL2Scancode(event3->key.scancode);
-        event2->key.keysym.sym = event3->key.key;
+        event2->key.keysym.sym = SDL3KeycodeToSDL2Keycode(event3->key.scancode, event3->key.key);
         event2->key.keysym.mod = event3->key.mod;
         event2->key.state = event3->key.down;
         event2->key.repeat = event3->key.repeat;
@@ -1902,18 +1920,48 @@ SDL_GetKeyFromScancode(SDL2_Scancode scancode)
     }
 
     if (scancode <= SDL2_SCANCODE_MODE) {
-        // They're the same
-        return SDL3_GetKeyFromScancode(SDL2ScancodeToSDL3Scancode(scancode), SDL_KMOD_NONE, true);
-    } else {
-        return SDL_SCANCODE_TO_KEYCODE(scancode);
+        /* Filter out extended keycodes. This matches the key mapping behavior of SDL2 backends. */
+        const SDL_Keycode keycode = SDL3_GetKeyFromScancode(SDL2ScancodeToSDL3Scancode(scancode), SDL_KMOD_NONE, true);
+        if (!(keycode & SDLK_EXTENDED_MASK)) {
+            return keycode;
+        }
     }
+
+    return SDL_SCANCODE_TO_KEYCODE(scancode);
 }
 
 SDL_DECLSPEC SDL2_Scancode SDLCALL
 SDL_GetScancodeFromKey(SDL_Keycode key)
 {
     SDL_Keymod modstate = SDL_KMOD_NONE;
     SDL_Scancode scancode = SDL3_GetScancodeFromKey(key, &modstate);
+
+    if (scancode == SDL_SCANCODE_UNKNOWN) {
+        /* Keys commonly remapped to extended keycodes in SDL3.
+         * If the keymap returns no mapping, convert the key to a scancode directly.
+         */
+        switch (key) {
+        case SDLK_LCTRL:
+        case SDLK_RCTRL:
+        case SDLK_LALT:
+        case SDLK_RALT:
+        case SDLK_LGUI:
+        case SDLK_RGUI:
+        case SDLK_CAPSLOCK:
+        case SDLK_SCROLLLOCK:
+        case SDLK_PRINTSCREEN:
+        case SDLK_INSERT:
+            scancode = (SDL_Scancode)(key & ~SDLK_SCANCODE_MASK);
+            break;
+        case SDLK_TAB:
+            /* ASCII value that can't be directly converted to a scancode. */
+            scancode = SDL_SCANCODE_TAB;
+            break;
+        default:
+            break;
+        }
+    }
+
     if (modstate != SDL_KMOD_NONE) {
         return SDL2_SCANCODE_UNKNOWN;
     }